Whether incorporated in video conferencing systems or standard speakerphones, the ability to communicate high quality Near-End signals while simultaneously receiving Far-End signals (i.e. full duplex communication) has proven to be a basic requirement of telecommunication systems.
An undesirable phenomenon inherent in full duplex, "hands-free", teleconferencing systems is that of signal echo caused by the acoustic coupling between a communication terminal's transducers. The echo in audio conferencing results from the re-transmission of a Far-End signal by the Near-End terminal of a communication system.
In speakerphones, the echo is caused by reflected Far-End voice transmissions which are coupled to the Near-End communication terminal's microphone section via the Near-End terminal's loudspeaker. Echo occurs with such audio conferencing systems because of the close physical proximity of the loudspeaker and microphone elements. The change in level of the echo as it is coupled from the Near-End's loudspeaker to the microphone is known as the Acoustic Echo Return Loss (AERL).
In order to reduce echo signals present in full-duplex conversation, several signal processing alternatives have been developed. Those alternatives include analog voice switching, echo suppression, and digital adaptive echo cancellation techniques.
High quality communication terminals often employ digital signal processing such as adaptive echo cancellation circuitry which predicts and synthesizes an expected feedback signal, and then subtracts the expected feedback signal from the Near-End microphone signal. Although adaptive echo cancellation provides significant reduction in echo signal levels, it does not eliminate echo signals. Moreover, such elaborate and often costly techniques are not economically feasible for all applications.
Even in applications utilizing echo cancellation circuitry, the performance of the adaptive echo cancellation circuitry is often affected by the strong coupling between the loudspeaker(s) and microphone. The coupling dominates the control process within the internal adaptive filter used by these communication devices, reducing the performance by limiting the maximum loudspeaker and microphone levels in order to reduce the acoustic echo to acceptable levels. Often times, users of such devices attempt to compensate this condition by increasing the volume beyond a limit which causes the device's software to revert to a "semi full-duplex" or half-duplex mode.
Therefore, it would be desirable to further reduce AERL prior to the electronic processing of the Near-End signal by a communication terminal to decrease the reliance on complex signal processing circuitry. In addition, it is also desired to reduce the level of feedback available to a communication device employing echo cancellation circuitry to offset limitations in the dynamic range of the adaptive filters of the circuitry.